I presently have a friction thumbshifter on the left and a cheapo five speed thumbshifter on the right. The latter only works because I'm using its clicks in an odd way; technically speaking the spacing is wrong, but the indexing mechanism will do 360 degrees, and there is a big gap between the #1 index and the #5 index second time around. I'm using the big gap for the 3-4 shift and the 2-1 index positions for the 3-2 shift if you see what I mean.
Both thumbshifters are mounted on a custom (OK... bodged up at home) welded bracket that mounts to stem nosebolt. I ride this bike on the tops a lot of the time so the levers are in the right place for me. If I do another one on a bike where I ride the hoods more it'll most likely have a set-up more like the one I made and posted pictures of in the 'skinflint' thread. Later single toggle five speeds can be controlled using a nice looking alloy thumbshifter which fits to 7/8" bars, but again a custom bracket of some kind may be required for a dropped bar installation. I think the alloy thumbshifter parts will mount to a conventional lever boss, but I've not BTDT so I can't say for sure.
The hub is only a tiny bit heavier than a three speed IIRC. The later (larger diameter) alloy hub shells are in fact heavier than the steel ones, and I have shown (well enough I hope) that you don't need the alloy flanges to stop spoke breakages. The alloy hubshells do look nice in a kind of chunky way though.
In use there are a couple of things that some people find off-putting; the freewheel sounds like your grandad's bike, and there is a fair amount of backlash when taking up the drive in some gears. Both these things are somewhat improved in later hubs but are not completely eliminated. All hub gears have some quirks of some kind I guess. I have mostly just got used to it.
As to 'that lightweight feel', 99% of the time this is dominated by the frame, wheels and tyres. Unless the hub rattles or something you don't notice it, it is not 'heavy' per se, and if there is a tiny difference in the weight distribution does this really matter? I doubt it.
Obviously more gears in a hub usually means more weight; at what point this automatically turns the bike into a heavyweight I don't know. A Nexus 7 is an altogether more sophisticated proposition in many ways; I've set my SA5 to give gear 3 at ~60-something inches; if you set gear 4 to be ~65" on a nexus 7 you get an extra low gear and an extra high gear vs the SA5, and if you set gear 5 to be a middle gear on a nexus 7 you effectively get two more low gears which I am sure some folk will value.
No one much (apart perhaps from me
) ever complained about having 'too many gears' but plenty of folk have complained at potentially reduced durability, excess weight, complexity, maintenance and associated expense; it is just a question of setting the priorities for your own use as to whether any given gear set-up makes real sense or not.